The invention relates to a printing roller for a rotary press, in which the printing plate in the form of a cylindrical wound plate is mounted rigidly on the outer surface of the printing roller by expanding a tubular, elastic, thin shell. The roller diameter is enlarged by the action of a non-compressible fluid on this shell from the inside, which action is controllable by devices for displacing the fluid.
Rollers for mounting tubular printing plates on rotary presses have already been disclosed, for example gravure plates, matrices for flexographic printing or for other enveloping bodies of a rotary press, such as, for example, rubber-like sleeves for contact pressure rollers, cylindrical printing plates for anilox rollers, casings for drive rollers and similar devices.
The known displacement devices, which are used for increasing the pressure in the fluid and for expanding the outer shell of the roller, have an actuating screw for generating a pressure force which acts directly on the fluid in the manner of a piston, frequently by interposition of a spring.
Due to the machining tolerances of the screw or threaded spindle, and of the threaded bore in the pressure line, pressure losses can arise which lead to a change in the fluid pressure and thus cause a change in the outer circumference of the shell. Another possible disadvantage is that the compression does not take place directly in the axial direction, which leads to further sealing losses and/or increased wear of the components. The use of a spring, together with a piston provided separately, requires the installation of three components, without eliminating the abovementioned disadvantages.
It is also necessary that the printing plate or, in some circumstances, the sleeve-like tubular envelope on the printing roller must be arranged with high accuracy in the axial direction of the roller, before expansion of the cylinder takes place, in order thus to avoid undesired shifts in the transverse direction. Furthermore, it must be taken into account that the edge regions of the known printing plate have sharp edges or edges angled off by 90.degree., which would disadvantageously affect the mode of operation of a wiping blade, that is to say that component which is provided for removing the ink excess and which moves to and fro in the longitudinal direction of the printing roller.
In particular when the printing plate was produced from a flat plate, it is always difficult to avoid sharp edges at the plate rims. As a result, the wiping blade is not only adversely affected in its oscillating movement and in its mode of functioning, but is also subject to heavier wear.
When a known printing roller is used in combination with a printing plate which consists of a flat metal plate coated with photosensitive material and wound in such a way that the printing plate assumes a cylindrical shape, a further disadvantage arises. Since the butt joint between the ends of the printing plate takes up a certain thickness, even though this thickening can be partially reduced, it is impossible to eliminate it completely.
When a roller is used which undergoes an enlargement in diameter, which is carried out in the conventional manner, a bulge or a projection extending in the longitudinal direction would result at the butt joint of the printing plate, which would lead to considerable difficulties in the use of this type of plate.
Alternatively, in order to avoid such enlargements at the butt joint of the printing plate, a different type of roller and a printing plate suitable for it have already been proposed, wherein the edges which have to be joined have a mutually complementary shaping which leads to a particularly complicated design of the edges, which is not free of restrictions.
The use of this known plate type is, however, particularly advantageous with respect to the manufacturing costs of the plate, both with respect to the necessary equipment and with respect to the time required for electroplating treatment and etching of these plates. However, the known advantages cannot be fully exploited, since an enlargement arising because of the mutual joining of the plate edges of these printing plates is formed at the butt joint on the inside of the cylindrical printing plate, because of the technical conditions described.